Most lightning protection systems for marine vessels are intended to provide a continuous conducting path from the point of the lightning attachment into the water in such a manner as to minimize damage to the hull, crew, and electronics. A typical marine lightning protection system has three major components namely: (i) one or more air terminals; (ii) one or more down conductors; and (iii) one or more grounding conductors, also referred to as ground, or grounding, electrodes. The air terminal is usually the highest point of the system and vessel and, therefore, where the lightning channel is likely to make its first connection to the lightning protection system on the vessel. The down conductors are connected to the air terminal and are designed to carry the bulk of the lightning current from the air terminal generally towards ground (or water). The grounding conductors are connected to the lower parts of the down conductors and are often immersed in the water so as to conduct the lightning current into the water. An example of such a system is described in American Boat and Yacht Council (ABYC) Standard E4 xe2x80x9cRecommended practices and standards covering lightning protectionxe2x80x9d (1999).
Some of the fundamental principles of lightning protection and a discussion of some problems with an earlier edition of the lightning protection code are presented in Thomson, E. M. (1991) xe2x80x9cA Critical Assessment of the U.S. Code for Lightning Protection of Boatsxe2x80x9d IEEE Transactions on Electromagnetic Compatibility 33(2):132-138. Ground currents are currents that flow into, and/or in, the water, usually through the ground electrodes. Sideflashes are discharges that form from conductors other than ground electrodes and frequently flow into the water. For example, sideflashes can form from conductors in the lightning system such as down conductors and bond conductors, as well as from conductors which are not part of the lightning system. Thomson, E. M. (1991) describes one origin of sideflashes as arising in response to potential gradients established in the water when the lightning current flows into the water through a grounding conductor. One serious problem identified by Thomson (1991) was the frequent occurrences of sideflashes in sail boats that were fitted with a lightning protection system. Specifically, in one sample 56% of 15 boats which were fitted with a lightning protection system and struck by lightning while in fresh water suffered from sideflashes that left holes through the hull of the boat. Thomson (1991) showed that these sideflashes could be predicted using the state-of-the-art model for voltages near a lightning ground (see as an example, Dwight, H. B. (1936), xe2x80x9cCalculation of resistances to ground,xe2x80x9d Trans. Am. Inst. Elec. Eng., 55:1319-1328). According to this model, voltages develop in response to current flowing into a resistive ground medium. For example, voltage differences between the lightning protection system, including all conductors attached to it, and a point in the water can result from an electric field established according to Ohm""s Law, J="sgr"E, where J is the current density, "sgr" is the resistivity of the water, and E is the electric field intensity. The voltage difference is then the line integral of the electric field between a point on the immersed ground conductor and the point in the water. The voltage difference between any conductor in the lightning protection system and that point in the water can be found in the same manner. If such voltage differences are sufficiently large, one or more sideflashes may develop.
As shown by Petropoulos, G. M. (1948) xe2x80x9cThe high voltage characteristics of earth resistancesxe2x80x9d JIEEE 95:59-70, the ground resistance of a single grounding conductor can be decreased if one or more sharp points are added to the grounding conductor, as sparks will form at the xe2x80x9cstrongly inhomogeneous electric field at the ends of their pointsxe2x80x9d. This is termed the xe2x80x9cdynamic groundxe2x80x9d effect. Thomson (1991) showed that a long strip conductor whose effective area is increased through the dynamic ground effect is preferable to a single ground plate. Subsequently, ABYC Standard E4 (1999), when discussing suitable geometries for a strip grounding conductor, states in E-4.9.1.2:
xe2x80x9cNOTES: 1. The edges of the external ground plate or grounding strip need to be sharp, exposed, and not caulked or faired into the adjoining area. 2. A strip approximately one inch wide and 12 feet long has nearly six times the amount of edge area exposed to the water, which, compared to the ground plates, will improve the dissipation of charges.xe2x80x9d
There are several patents relating to lightning grounding in a boat. See, for example, U.S. Pat. No. 5,036,785 issued to Kittredge et al., which also refers to other prior art devices (U.S. Pat. No. 11,217 issued to Forbes; U.S. Pat. No. 2,909,589 issued to Booker; U.S. Pat. No. 3,483,305 issued to Bonkowski et al.; and U.S. Pat. No. 3,919,956). In addition, U.S. Pat. Nos. 6,029,597 and D425,481 issued to Cutler describes a lightning discharge strip with a xe2x80x9cplurality of parallel groovesxe2x80x9d to xe2x80x9cmake advantageous use of edge technology to dissipate electrical charges caused by lightning strikesxe2x80x9d.
Another device is the Strikeshield xe2x80x9cdissipater electrodexe2x80x9d developed by SEYLA Marine. These prior art inventions are designed to minimize ground resistance and are designed to be completely immersed, with the exception of parts used for mounting.
Another device, for transient protection, is described in U.S. Pat. No. 3,818,259 and is related to a spark gap device. Typical spark gap devices can have two or more stationary electrodes which form a spark connection when subject to a large enough voltage difference between the electrodes. This concept has also been applied to transient grounding as in the TEC100C Transient Earth Clamp manufactured by ERITECH. The TEC100C operates as an open circuit except during a lightning transient when it clamps shut.
Typically, lightning grounding systems for boats incorporate grounding conductors which are completely immersed in the water during a lightning strike, except for parts that are intended for attachment to the hull or mast. The effectiveness of such grounding conductors is gauged by ground resistance. This ground resistance may have a dynamic component. However, as Thomson (1991) points out, a single ground plate or strip may need to be supplemented by additional grounding conductors distributed over the hull surface in order to prevent sideflashes.
The subject invention pertains to a method and apparatus for grounding or redistributing lightning charges. The subject invention can utilize the promotion of sparks from electrodes to replace, or compliment, conducting connections. In a specific embodiment, a grounding electrode can be connected to a down conductor and the electrode can be embedded in the hull of a marine vessel below the waterline. Such electrode can promote spark formation with the objective of providing protection against damaging sideflashes. The electrode can be recessed below the hull surface and can be faired into the hull to decrease drag. Such fairing can reduce galvanic or electrolytic corrosion. The shape of the grounding electrode can be designed to enhance the electric field between the electrode""s tip and the water upon the lightning protection system becoming charged as a result of lightning or other atmospheric phenomenon. In operation, the electrode can preferentially launch a spark towards the water to prevent sideflashes from being initiated by other charged conductors inside or on the vessel. After the formation of a spark, the subsequent arc discharge can change the local electrical environment so as to lower the ground resistance and further decrease the likelihood of sideflashes.
In another embodiment, the subject electrode can be embedded in, and electrically connected to, a metal ballast or ground plate below the waterline, where the ballast or ground plate is electrically connected to a down conductor. The subject electrode can then promote spark formation and current flow from the ballast or ground plate into the water. In another specific embodiment, a set of two or more electrodes can enable a grounding connection to be established through a tank holding a conducting fluid such as water. One or more upper electrode(s) can be connected to a down conductor, while one or more double-ended electrode(s) can be embedded in the lower portion of the tank below the fluid in the tank.
In another specific embodiment, an electrode can be connected to a down conductor or bonding conductor and embedded in the hull above the waterline of the vessel. In a further embodiment, an electrode connected to a down conductor system can promote sparking to a rotating conductor that may be partially submerged, as for example, in the case of a propeller shaft. In an additional embodiment, an electrode can promote a spark to a bonding conductor to form a dynamic bonding connection.